Numerous manufacturing and processing methods generate waste water that cannot be disposed of via the sewers because of their composition, and that contain substances the recovery and re-use of which could be economically gainful. One example is the photographic processing industry, in which exposed films and photographic papers are treated in successive processing baths containing large numbers of chemicals. Such methods for processing photographic films are well known (see for example, Chimie et physique photographiques, Pierre Glafkides, Vol. 2, ch. XL, pages 947-967), and therefore require no further description. These processes produce washing water containing relatively low concentrations of chemicals that are costly to remove by current methods.
In a first established approach, the treatment of waste water from photographic baths takes place in two steps, one step to eliminate salts from the solution by ion exchange, and one step to eliminate organic chemicals by absorption e.g., using activated carbon. Using a subsequent process involving additional chemicals, the substances extracted from the solutions have then to be removed from the ion exchange resins and the activated carbon.
Evaporation and distillation are also used to separate dissolved substances. However, for very dilute solutions, these processes are costly because of the high energy consumption they entail.
In a second more recent approach, ultrafiltration, nanofiltration, and reverse osmosis have been used for waste water treatment. In this approach, each treatment bath in a processing plant is linked to its own ultrafiltration or nanofiltration unit. Such units use membranes, which behave in principle as large surface-area sieves, the "holes" of which are pores of microscopic or molecular dimensions, the size range of which must be very narrow so that molecules greater than a set size are retained while smaller molecules and simple salt ions are let through the membrane. The membranes for ultrafiltration generally let through molecules with molecular weights less than about 2,000, larger size molecules being retained. In nanofiltration, this molecular weight threshold is about 200. The molecular weight threshold for reverse osmosis is about 100 or less. In this description, the term "filtration" refers indiscriminately to ultrafiltration, nanofiltration or reverse osmosis, i.e., all systems of filtration by membrane technology.
Filtration membranes of this type can possess high selectivities, but they allow only low flow rates. In general, one filtration unit is used per treatment bath, i.e., one unit to treat the waste water from the developing bath, a second one for the fixing bath, a third one for the bleaching bath, and so on. The permeate from each of these filtration units is recycled exclusively to the washing bath that is associated with the bath the waste water came from. Such systems are abundantly described in the patent literature, in particular in Patents U.S. Pat. No. 4,451,132 and FR-A-2 684 024.
The main drawback of these arrangements is that the large number of separate ultrafiltration or nanofiltration units increases the cost, space requirements, and maintenance needs of the processing plant.
In addition, the substances that contaminate the washing water from photographic processing are very diverse; they include organic compounds such as developing agents, inorganic chemicals, in particular mineral salts, and chelates. All these substances have to be removed, so the membranes have to be chosen and used in such a way that all these substances are eliminated completely, or at least to a degree that meets the photographic processing standards in the case of recycling, or effluent standards. However, if the waste water is strongly demineralized, the resulting water is no longer able to fulfill its washing function when it is recycled in the photographic process, and yet if it is not thoroughly rid of contaminants it cannot be recycled indiscriminately at any step in the process.